The primary focus of the present invention is sensitive and fast detection of infrared signals, with particular emphasis on imaging arrays. Until recently, two main classes of infrared photoconductive detectors have been under investigation. These either employ band-to-band transitions or dopant-to-band transitions. The band-to-band detectors usually require semiconductors whose band edge is close to the wavelengths to be measured. In the infrared, such detectors are typically fabricated in materials which are difficult to make into imaging arrays, such as HgCdTe. Alternatively, the detectors requiring dopants operate at very low temperature. For this reason there has been a search for new detectors.
Recent advances designed to overcome the limitations of either band-to-band or dopant-to-band detector have used intersub-band transitions. The first example is in quantum wells using the intersub-conduction band transitions first reported by L. C. West and S. J. Eglash, Applied Physics Letters, Vol. 46, pp. 1156-1158 (1985). This detector uses quantum wells to provide discrete energy level subbands for the electrons. Transitions take place within subbands. B. F. Levine et al, Applied Physics Letters, Vol. 56, pp. 851-853 (1990) have improved the quantum well infrared photodetector by using the confined energy level to unconfined miniband transition. The unconfined miniband lies above the barrier level. With this transition, the detectivity of the detector has reached as high as 1.times.10.sup.10 cm Hz.sup.1/2 /W. However, this detector has a serious limitation, in that because of selection rules, as described by D. D. Coon and R. P. G. Karunasiri, Applied Physics Letters, Vol. 45, pp. 649-651 (1984), light incident from above onto the detector is not absorbed. Hence these detectors require an end-fire coupling, or a corrugated surface (for grating coupling), which automatically reduces their sensitivity. Furthermore, even with proper optical coupling, there is automatically a reduction of a factor of two in quantum efficiency because of the selection rules which utilize only one polarization of light. An additional problem for this detector is that the dark current is extremely large.
B. F. Levine et al, Applied Physics Letters, Vol. 59, pp. 1864-1866 (1991), proposed and demonstrated a detector that is sensitive to light incident from above. This detector uses intersub-valence bands; that is, transitions between the occupied confined heavy hole and the unoccupied continuum states.
These two detectors are compared in Table I below, which provides a comparison of the responsivity, gain, quantum efficiency, absorption coefficient of the quantum well, mean free path of the carrier, dark current, D*, and geometry for the 40 .ANG. n-doped GaAs/Al.sub.0.3 Ga.sub.0.7 As intersubconduction band detector (West and Eglash, supra; Levine et al, 1990, supra) at 10.7 .mu.m at 4 V and a 40 .ANG. p-doped GaAs/Al.sub.0.25 Ga.sub.0.75 As intersub-valence band quantum well detector (Levine et al, 1991, supra) at 8.4 .mu.m at 5 V. The size of both detectors was 200.times.200 .mu.m, with 50 quantum wells each.
TABLE I ______________________________________ Comparison of the Properties of Two Prior Art Detectors. 40 .ANG. p-doped 40 .ANG. n-doped Intersub-valence Intersub-conduction Band Quantum Band Quantum Well Detector Well Detector ______________________________________ Responsivity (A/W) 0.039 1.0 Gain 0.024 0.8 Quantum efficiency 28% 20% Absorption Coeffi- 8,212 13,000 cient (1/cm) Mean free path (.ANG.) 408 13,800 Dark Current (A) .apprxeq.1 .times. 10.sup.-7 A .apprxeq.3 .times.10.sup.-5 A D* (cm Hz1/2/W) 1.7 .times. 10.sup.10 4.0 .times. 10.sup.10 Geometry Normal Non-normal ______________________________________
The advantage of the intersub-valence band detector is that it can be used with normal incident light. The problem with the intersub-valence band detector is that its responsivity and gain are low. This is the result of the heavy hole being much more massive than the conduction electron, which causes a very short mean free path of the heavy hole after it leaves its potential well. In the barriers, the heavy and light hole states are degenerate, which decreases the lifetime of the hole carriers, since scattering between states insure some heavy hole nature in the carriers. It is this decreased mean free path and lifetime which causes the gain and responsivity much lower than the intersub-conduction band quantum well detector. Although the dark current decreases by a factor of 300, further reduction in the dark current is desirable.
Thus, what is desired is a quantum well detector that combines the advantages of intersub-valence band detectors, namely, the use of incident light normal to the detector, with the higher responsivity and gain of the intersub-conduction band detector. In addition, the dark current must be reduced.